For example, EP 0219360 and U.S. Pat. No. 5,259,733 use two electrical switches operated by two magnets, the fields of action of which, when brought together, disturb the functioning of the machines. EP 0321376 uses an electric switch actuated by two magnets, which does away with one switch but still requires two magnetic fields, also brought together. The present device by contrast attains a high degree of simplicity, with the result that, it is much more reliable and inexpensive; also, other disadvantages of the devices of the prior art are eliminated with the present device, which also offers other advantages that will become clear in the following text.